Artist rendering of NASA's Kepler. The mission will spend three and a half years surveying more than 100,000 sun-like stars in the Cygnus-Lyra region of our Milky Way galaxy. It is expected to find hundreds of planets the size of Earth and larger at various distances from their stars. If Earth-size planets are common in the habitable zone, Kepler could find dozens; if those planets are rare, Kepler might find none. less

Artist rendering of NASA's Kepler. The mission will spend three and a half years surveying more than 100,000 sun-like stars in the Cygnus-Lyra region of our Milky Way galaxy. It is expected to find hundreds of ... more

Photo: David Koch, NASA

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William Borucki, chief scientist on the Kepler space mission, discusses the project at NASA Ames Research Center in Mountain View, Calif., on Wednesday, June 24, 2009. The Kepler spacecraft is orbiting the sun in search of distant earth-like planets. less

William Borucki, chief scientist on the Kepler space mission, discusses the project at NASA Ames Research Center in Mountain View, Calif., on Wednesday, June 24, 2009. The Kepler spacecraft is orbiting the sun ... more

Photo: Paul Chinn, The Chronicle

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NASA extends planet-hunting Kepler mission

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Astronomers hunting for unseen planets far beyond our solar system have been authorized to continue their 3-year-old quest for another four years, NASA officials announced Wednesday.

A committee of independent scientists reviewing the work of the Kepler mission called it an "outstanding success" and said it has produced "a continuing stream of new findings" about the existence of faraway solar systems in the Milky Way galaxy.

Based at NASA's Ames Research Center in Mountain View, the Kepler spacecraft's telescope will now continue surveying 170,000 stars in the Cygnus and Lyra constellations, looking for the tiny dimming of a star's brightness that would signal the passage - called a transit - of an orbiting invisible planet.

Since the spacecraft's launch in November 2009, scientists keeping vigil over its telescope have identified 2,300 "candidate" planets and have now confirmed that 61 really are planets, according to William J. Borucki, the physicist and leader of the international Kepler team.

To astronomers around the world, all objects orbiting stars beyond the solar system are known as exoplanets.

Borucki said that among Kepler's new-found exoplanets, 38 orbit their stars within what astronomers call their habitable zones - distances from their stars that are just right for liquid water, and perhaps even life, to exist.

Before scientists confirm that each candidate is a true exoplanet, they must follow it for at least three transits across the face of its star to prove that it's really in orbit around the star.

NASA's decision on Kepler not only extends the mission but will allow for more detailed confirmation of each object's planetary status.

"Now, we'll have time to observe five or six - or even eight transits," Borucki said, "and that should tell us much more - whether it's a rocky one like the Earth or a gas giant like Jupiter."

The Kepler mission's cost was originally about $500 million for the satellite, its telescope and the first 3 1/2 years of operation. Its budget will now be about $18 million a year for the next four years.

The "senior review" of outside scientists that approved continuing the Kepler mission also approved the 22-year-old Hubble Space Telescope, whose spectacular images of cosmic events have revealed insights into the birth of stars and the violence of distant galaxies.

It also approved continuing operation of the lesser-known 12-year-old Chandra X-Ray Observatory, a NASA "flagship" mission that has revealed some of the most extraordinary events in astronomy. These include supermassive black holes, neutron stars and exploding stars known as supernovae whose ages have offered new clues to the Big Bang itself.